avoid resetting framerate during vpx_codec_enc_config_set()

avoid resetting framerate during vpx_codec_enc_config_set()

The calculated frame_rate is a state variable in the codec, and
shouldn't be maintained in the configuration struct. Move it to the
main part of cpi so that it isn't clobbered when the configuration
struct is updated. The initial framerate estimate is moved from the
vp8_cx_iface.c wrapper into the body of init_config() in onyx_if.c, so
that it is only called once and not reset on every call to
vp8_change_config().

Change-Id: I8d9a3d1283330d1ee297d07e9d78d1f2875f2465